Geochemical prospecting



p 1943- M. A. ARTHUR 2,330,021

GEOCHEMICAL PROSPECTING Filed NOV. 25, 1940 md/u'an d. M INVENTOR.

BY W 9 6 reproducible with such samples.

grain size is obtained by drying the sample, re-.

Patented Sept. 21, 1943 GEOCHEMICAL PROSPECTIN G Marion A. Arthur, Houston, Tex, assignor to Standard Oil Development Company, a. corporation of Delaware Application November 25, 1940, Serial No. 366,952

3 Claims.

The present invention is directed to geochem ical'prospecting for petroliferous deposits, and particularly a prospecting method in which soil samples and soil water samples are examined for their content of selected minerals.

It has already been proposed to prospect for oil by collecting samples of soil and analyzing individual samples for their content of a selected inorganic ion or radical. Samples are analyzed for their content of sodium, potassium, calcium, chromium, magnesium, chlorine, sulfate ion, sulfate radical, and the like.

Due to the fact that many of these ions or elements are present in soil samples in only infinitesimal amounts, some difiiculty is encountered in determining the slight differences in the ionic content by chemical analysis. Accordingly, analysis by the emission spectrum was resorted to.

It has been found that, in order to conduct a reliable survey by analyzing soils for a selected inorganic constituent by the emission spectrum analysis, certain precautions have to be observed. Chief among these is the grain size of the sample to be analyzed. Extensive experience has demonstrated that the grain size of. the samples should be maintained within certain limits in order that uniform and comparable results may be secured. Reliable results are obtained when the grain size of the samples is between 147 and 417 microns. When the grain size is larger than 417 microns, it happens that the are often becomes concentrated on one particle and does not give a true picture of the composition of the sample being analyzed. It is not clearly understood what causes the difliculty when the particle size is too small, but experience has demonstrated that results are not readily The desired ducing it to powder, then collecting that portion which passes through a 40 mesh screen, but does not pass a 100 mesh screen.

ume and filling the craters in each case exactly level. The size may be maintained constant by weighing, but this is a painstaking operation and there is very apt to be loss in the handling of the weighed sample which introduces errors into the analysis. By using uniform sized craters and .must be always the same.

filling them exactly level in each case, the size requirement is readily observed.

Of course, it will be understood that for comparable results it is also necessary to use a uniform exposure time for the arcing of all samples and to utilize the same arcing current in all cases. Likewise, the gap between the electrodes Other precautions which are common in this art are that the slit width of the instrument must be uniformly the same and the developing time and developing temperature must not be changed from sample to sample. It will be understood that reference is had to the developing of the picture of the grating which is made for each sample.

It has also been found that uniformity and reliability of results is improved bymaintaining a uniform rate of flow of air through the arc gap. This currentof air performs a dual function of carrying of! combustion products which might contaminate the electrodes and lead to erratic results, and also of maintaining the temperature within narrow limits, which also contributes to the reliability of the results. The current of air, of course, should not be sufliciently rapid to distort or extinguish the arc. A suitable velocity has been found to be one which will exert a pressure of three inches of water above atmospheric pressure on the exhaust line.

A suitable arrangement for maintaining the air current is shown in the accompanying drawing in which the single figure is a side elevation of an arc housing with the vacuum line attached according to the present invention. In this drawing no effort is made to show the details of the arcing circuit, since this is conventional equipment.

Referring to the drawing, numeral l designates an arc house supported by a standard 2 and having a door 3. Extending upwardly through the bottom of the house is a rotatable shaft 4 having a turning knob 5. As is known on the upper end of this shaft is a table or tray carrying a plurality of electrodes circumferentially spaced, each of which has a cavity to receive a sample to be analyzed and is adapted to be brought into alignment with an electrode 6 extending into the arc house from the top through a bushing l. The shaft 4 is of such a size that air can enter the arc house around it. Fixed in the top of the arc house is an air outlet 8 which is connected to a vacuum pump 9 and also to a manometer It. A valve H is arranged in line] to'regulate the amount of suction on the arc house so as to maintain the pressure at any correlation-of the samples desired level. It is to be understood that a plurality of outlets t can be arranged around the top of the arc house.

In the practice of the present invention, therefore, soil samples are collected at spaced points over an area to be explored. The samples are usually collected along traverses laid out across the area to be investigated in such a way that the number of samples will be representative of the area. Usually the samples are collected about a tenth of a mile apart. The samples are collected at a depth of several feet. Uniformly reliable results have been obtained by collecting samples at a depth of ten feet. When possible, the collection depth should be maintained uniform over any given area. Useful results have been obtained, however, with samples collected at a depth of ten inches. In practice, it is preferred to collect the samples at a depth of at least four feet. In well logging the samples are generally cuttings which are picked up in'the mud ditch and correlated with depth. Such cuttings can be collected at any selected intervals, such as at every thirty feet or some multiple thereof. This figure is convenient because each section of drill pipe is thirty feet in length and the depth expedient.

The samples are carefully dried and then powtiered and screened. Particles which pass the 40 mesh screen and are collected by a 100 mesh screen are then loaded into the crater of an electrode in such a way as to fill it exactly level full. the camera is exposed to the reflection of the are from the grating for a predetermined period of time. Usually the film is immediately developed, each film being subjected to the same period of developing time at the same temperature. The intensity of any selected line on the film, representing a selected element, is then determined in the usual manner byplacing the film between a source of-light and a photocell and reading the current in the photocell circuit. The lines on the film to be examined for various elements are listed in the tables of wave lengths of the principal lines in the emission spectra of the elements in any standard hand book as, for example, on pages 194 et seq. of the 18th edition of Hodgmans Handbook of Chemis simplified by this The are is then started and the film in istry and Physics. The-quantity of the element or elements analyzed for is then noted on a map or chart in correlation with the sample location.

The nature and objects of the present invention having been thus described and illustrated, what is claimed as new and useful and is desired to be secured by Letters Patent is:

l. A method for .prospecting for subterranean petroliferous deposits which comprises collecting samples of soil at spaced points in an area to be explored, drying each sample, crushing and screening it so as to produce particles having a particle size between about 147 and about 417 microns, and analyzing such particles for a selected inorganic constituent by emission spectrum analysis whereby the analyses may be correlated with sample locations to produce a picture from which the presence of subsurface deposits of oil may be prognosticated.

2. A method for prospecting for subterranean petroliferous deposits which comprises collecting samples of soil at spaced points in an area to be explored, drying each sample, crushing and screening it so as to produce particles having a particle size between about 147 and about'417 microns, and analyzing the same volume of said particles of each sample for a selected inorganic constituent by emission'spectrum analysis whereby the analyses may be correlated with sample petroliferous deposits which comprises collecting samples of soil at spaced points in an area to be explored, drying each sample, crushing and screening it so as to produce particles having a particle size between about 147 and about 417 microns, subjecting the same volume of said particles of each sample to the arc of an emission spectrograph, maintaining a flow of air of the same velocity through said are while each sample is being arced, and producing an emission spectrum of each sample whereby its content of a selected inorganic constituent may be' determined whereby the-spectra so obtained may be correlated with sample locations to produce a deposits of oil may be prognosticated.

MARION A. ARTHUR. 

